In many areas of semiconductor processing, it is often necessary to provide consecutive layers of materials that are not stable in contact with each other. For example, aluminum (Al) reacts with silicon at a few hundred ° C. to form “spikes” of an eutectic alloy which can penetrate into the silicon through the source or drain layer causing shorts to the body if a direct Al—Si contact is made. Additionally, silicon (Si) must also be protected during tungsten deposition, as the copious amounts of fluorine present will combine with hydrogen to form hydrofluoric acid (HF), which can attack silicon or silicon dioxide to form “wormholes” under the tungsten layer. Furthermore, copper (Cu) used in IC metallization must not encounter silicon dioxide passivants, as Cu+ ions will diffuse readily through the oxide and contaminate the underlying silicon.
In all the above cases and more, the situation is rescued by employing barrier materials, which are typically metals or nitrides of such metals in most applications that conduct electricity but do not permit interdiffusion and reactions of neighboring materials. However, certain barrier materials exhibit tensile or compressive stress when deposited as a thin film. In some cases, stress will build up because of the processing conditions, thermal expansion, or the mismatch of various characteristics of neighboring materials. As an example, low-Cl and low resistivity TiN films (TiCl4-based) exhibit very high in-film tensile stress when deposited on a silicon substrate.
The conventional method of depositing such thin films includes continuous deposition of a barrier material onto a semiconductor substrate until the desired thickness has been attained. The prior art continuous deposition method results in a structure as seen in prior art FIG. 1: a thin film of the barrier material 10, deposited onto a semiconductor substrate 12. When the thin barrier film made using the prior art process consists of TiN, for example, the thin barrier film 10 after heat treatment exerts a tensile stress 14 on the underlying substrate 12. The force exerted in compressive stress 16 by the substrate 12 is equal and opposite in directionality to the tensile stress 14 of the thin barrier film 10.
If the magnitude of the in-film tensile stress is sufficient, the thin barrier film 10 may crack, buckle, delaminate or pull away from the surface of the substrate 12, or even cause stress-related breakage of metal interconnects limiting applications of the used barrier material. This in-film stress also limits the thickness of such thin barrier films in applications because thicker films have more potential energy to crack and peel. Additionally, high stress levels in such thin barrier films can affect many material properties such as dielectric constant and crystallographic orientation. These damaging effects may occur during the course of the integrated circuit manufacturing process, or at any time throughout the useful lifetime of the integrated circuit device, resulting in yield loss and seriously affecting the reliability of the product seriously.
It would be, therefore, desirable to provide a method of depositing thin barrier films on semiconductor substrates in a manner that addresses in-film stress such that the thin barrier films exhibit reduced tensile or compressive stress following deposition.